fix(p2): cancellable/bounded async task lifecycle (FTS loop, forge3d accept, telemetry spawns)

[KooshaPari]

Summary

• forge_api FTS loop (crates/forge_api/src/forge_api.rs): replaced fire-and-forget tokio::spawn with a tracked JoinHandle and CancellationToken. The loop select!s on shutdown so it exits immediately rather than waiting for the next interval tick. BackgroundTasks owns the handle and is embedded in ForgeAPI; tasks are aborted on drop.
• forge3d accept loop (crates/forge3d/src/server.rs): serve() now takes a CancellationToken and select!s on it in the accept loop. Per-connection spawns are tracked in a JoinSet (not fire-and-forget); on cancellation the set is aborted and drained.
• Convention documented in both files via // Task-lifecycle convention (P2.4) comments.
• forge_dbd untouched (owned by lane P2.3).

Test plan

• serve_exits_on_cancellation — new test verifying serve() returns Ok(()) within 2 s of token cancellation
• cargo test -p forge3d -p forge_api — 28/28 green
• cargo build (full workspace) — exit 0
• cargo clippy -p forge3d -p forge_api — clean

🤖 Generated with Claude Code

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❌ Phenotype Agent
❌ KooshaPari
❌ Dmouse92

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[KooshaPari]

Wrong base (upstream); intra-fork for KooshaPari/forgecode. Reopening there.